This invention relates to a system for driving hydraulic actuators from electronic control signals and, more particularly, to an integrated combination of an electric motor and hydraulic pump within a common housing.
One of the design objectives in the next generation of commercial, and possibly military, aircraft is the replacement of conventional hydraulic drive systems for hydraulic actuators which determine the positions of movable parts such as rudders, ailerons, elevators, landing gear and the like with remote electric motor/pump units located in the vicinity of the individual hydraulic actuators. Such a so-called "fly-by-wire" system is expected to provide substantial benefits for future aircraft in terms of reliability, weight reduction, control signal response, reduced cost, and simplification of the control systems.
The hydraulic control systems which have been used on larger aircraft for many generations have depended upon one or more engine driven hydraulic pumps and numerous hydraulic lines extending from the hydraulic pressure source via a central control mechanism in the cockpit to the various actuators in the wings, tail and body of the aircraft. These hydraulic systems have been prone to leak and have also been subject to operation problems resulting from old or contaminated hydraulic fluid, dirt or bubbles in the lines, pump failure, etc. There have even been occasions where aircraft have been lost because of a catastrophic hydraulic system failure.
It is considered that if electric motors and pumps for generating the required hydraulic pressure for the hydraulic fluid actuators are provided with the actuators on an individual basis, the elimination of the plurality of hydraulic lines extending from the cockpit to the various actuators and the need for a central hydraulic pressure source will bring about a substantial reduction in weight and the elimination of many of the problems now related to the conventional control system. The introduction of an electrical control system for the motor/pumps associated with the individual hydraulic actuators not only results in more effective control of the hydraulic actuators but also simplifies the task of transmitting the signals from the pilot's controls to the individual actuators.
The available space for a motor pump combination in the location of a hydraulic actuator on aircraft is somewhat limited, particularly in the locations of the actuators for ailerons and elevators. Therefore, it is preferred to utilize an integrated design in which the pump elements and motor housing are co-extensive with one another. Such designs are known in the prior art, one such being disclosed in U.S. Pat. No. 3,295,457 of Oram. The Oram unit comprises an electric motor, the rotor of which houses a plurality of cylinders with a piston in each cylinder, each piston having one end projecting from one end of the cylinder. The central shaft of the unit is fixed in position and the rotor, including the cylinders and pistons, rotates about the fixed central shaft. The unit contains an angled thrust plate, sometimes called a "swash" plate, which is fixed at an angle to the axis of the shaft and rotor. The projecting ends of the pistons are constrained to be maintained in sliding relationship with the thrust plate, thereby reciprocating within their cylinders as the cylinders rotate about the central shaft axis. This reciprocating motion of the pistons serves to pump the hydraulic fluid through the system.
This type of integrated motor pump design is not without its problems, however. Since the space within the unit in which the rotor rotates is filled with fluid, the electrical components of the motor may be subject to damage, particularly if the fluid being pumped is corrosive in nature, unless steps are taken to protect them from exposure to the hydraulic fluid. For the precise control required in the particular application of the invention which is described herein, some arrangement is required which provides precise angular control of rotor position to a higher degree than the Oram unit is capable of. Other improvements over like devices of the known prior art are also provided by the present invention.